A bootable flash drive is a flash drive that boots up when you start your system. Having a bootable flash drive comes in handy if you ever want to install an operating system on your flash drive to use for emergency purposes. With it you can back up critical data, transport large files, rescue a malware-infested PC, and even run an entire operating system.

For example, I recently spent days troubleshooting a relative’s malware-infested, effectively non-operational laptop. She wasn’t concerned about getting the system working again, but she desperately wanted to retrieve her business documents, personal photos, and the like.

There is howto’s:
Step 1: Just download and run the program, then choose a Linux distribution (i.e. a version of Linux) or, if you already have one, an ISO file. I think most people will opt for the former, at which point UNetbootin downloads Linux for you, then copies it to your flash drive (and makes the drive bootable).
Note: StIf you’re new to Linux, I recommend choosing Ubuntu–though readers may have other ideas as to which distribution is the most user-friendly.

Step 2: Reboot with your drive still inserted and presto: Linux should load. (You may need to monkey with your BIOS or boot settings so that the flash drive is at the top of the order.)

Step 3: Keep in mind that this won’t overwrite anything on your hard drive; the OS runs exclusively from the flash drive. When you’re done, you can just shut down, remove the drive, then boot back into Windows normally.